The present invention relates to a device for feeding the electrode or electrodes of a signle or multiphase electrothermal furnace by means of a main transformer and an additional transformer, with the primary winding or windings of the main transformer being supplied with the mains voltage, the device further including a first secondary winding for transforming the primary voltage for the electrode and a second secondary winding for making available an adjustable additional voltage, with this additional voltage being summed with the primary voltage by means of the additional transformer.
In the past, the additional voltage has been set by means of a load stage switch which, in each stage, taps a certain amount of the voltages induced in the second secondary windings.
The present invention can be used in electric arc furnaces and also in reduction furnaces.
The amount of steel produced in electric arc furnaces is increasing steadily. However, to realize the most economical production, i.e. thermal efficiency, compared to oxygen converters for the production of mass-produced steels, furnace capacities and transformer terminal power have been increased steadily. A significant prerequisite for the operation of large furnaces, however, is a sufficiently strong mains, so as to substantially exclude reactive effects, e.g. voltage fluctuations, asymmetries, on the mains affecting the general energy supply and its consumers. If the mains short-circuit power is not great enough, cost-intensive dynamic compensation systems must be installed and connected in parallel with the electric arc furnace so as to compensate for fluctuations in reactive load.
These reactive load fluctuations occur particularly during the melting phase, caused, inter alia, by scrap sliding in and the single-phase, two-phase or three-phase short circuits and current interruptions associated therewith.
The most recent developments are directed toward the attainment of more favorable operating conditions with the aid of thyristor controlled direct current furnaces. Measurements have shown that these furnaces can significantly reduce mains voltage fluctuations.
However, a significant drawback of such furnaces is primarily that they usually have only one electrode so that the counterelectrode which must receive the return current to the thyristor system is disposed at the bottom of the furnace.
Complicated cooling of the bottom electrodes and time and cost consuming replacement of the bottom electrode are associated therewith.
In contrast to a three-electrode furnace, fluctuations in active power in the above-mentioned single electrode furnace are very distinct when the arc is quenched, between zero and maximum active power. This discontinuous use of power will have a negative effect primarily on productivity, as regards loss of melting times, and results in stresses on furnace and mains, including current surges.
High investment costs, brought about primarily by rectifier systems which must be designed for full furnace power, prevent, inter alia, the construction of larger units.